Surface coatings containing effect pigments, such as light absorbing pigment, light scattering pigments, light interference pigments, and light reflecting pigments are well known. Metallic flake pigments, for example aluminum flakes, are examples of such effect pigments and are especially favored for the protection and decoration of automobile bodies, such as for example by reason of their imparting a differential light reflection effect, usually referred, to as “flop”, as well as flake appearance effects, which include texture, sparkle, glint and glitter imparted by the flake as well as the enhancement of depth perception in the coating. The appearance effects are a function of the flake type, flake size distribution, surface smoothness, orientation, and uniformity of flake edges. Metallic coatings usually also contain pigments, generally of a light absorbing rather than a light scattering type. These light absorbing pigments interact with effect pigments such as flakes to change the appearance effect of the coating. In general, visual coating appearance includes texture, sparkle, glitter or other visual effects of a coating. The visual appearance can vary when viewed from varying viewing angles, with varying illumination angles, or with varying illumination intensities.
Traditionally, coating appearance is measured based on human eye judgment. Some attempts were made to measure appearance of a target coating using instruments. U.S. Pat. No. 6,952,265, disclosed a method to obtain appearance data of a target coating in digital images and correlate the appearance data to appearance characteristics stored in a coating database. However, it requires to: (1) obtain images of the target coating at multiple illumination intensities and (2) measure the images at the multiple illumination intensities and at multiple threshold levels. U.S. Pat. No. 6,975,404 disclosed a process and device for measuring appearance properties of a coating having reflective flakes. However, it requires measurement at multiple illumination angles.
It is therefore still in need for a simple method and system for measuring and comparing coating appearances that are without the need for measuring appearance at multiple illumination intensities and angles and substantially independent from human visual judgment.